Comments on: is_transparent: How to search a C++ set with another type than its key

By: Jonathan Boccara

Jonathan Boccara — Sun, 07 Oct 2018 15:22:00 +0000

In reply to Kupiakos. Do you mean using a different hash function for searching than the one that is in the container? Which hash function would you use for the insertions?

By: Kupiakos

Kupiakos — Fri, 28 Sep 2018 00:15:00 +0000

I wonder if there’d be a good way to add something like this to std::unordered_map – searching by a hash or with a different hash function maybe?

Nitpick: using he/him to refer to “default” programmers is a pet peeve of mine – society’s come far enough to use gender neutral English

By: Paul Cook

Paul Cook — Sat, 10 Jun 2017 09:29:00 +0000

Very interesting and if timely use for me, thanks. I shall dig further.

By: Daniel Houck

Daniel Houck — Fri, 09 Jun 2017 22:14:00 +0000

In reply to Hanna Khalil. The solution to the problem is that C++14 introduced the

template< class K > iterator find( const K& x );

overload (and a similar const one), but because of template metaprogramming they only work if

typename Compare::is_transparent

is valid. I believe the reason they made it require that tag is to avoid breaking old code where the comparison would do a different conversion than changing the K to a Key, but I don’t know the details.

By: Daniel Houck

Daniel Houck — Fri, 09 Jun 2017 15:49:00 +0000

This is interesting and useful; I’d seen references to `is_transparent` before but always worded in a hard-to-understand way I hadn’t yet tried to parse out.

However, the most interesting thing I got out of this article is that you can inherit from lambdas to make overloads of lambdas, which is *really cool* and I hope that C++20 has a `std::function_overload` type in “, or equivalent.

By: Hanna Khalil

Hanna Khalil — Fri, 09 Jun 2017 13:16:00 +0000

Nice presentation of the problem but it’s not explained how the proposed is a solution to the problem

By: Dan Ibanez

Dan Ibanez — Fri, 09 Jun 2017 13:06:00 +0000

This feature was sorely needed in my opinion. I’m a bit confused as to why they decided to go with an ignored typedef is_transparent, but it seems straightforward enough to use anyway. Thank you for writing about this, I hadn’t heard of it before.